In a motor with a permanent magnet rotor, the operating speed of the motor is partially limited by the difficulties of retaining the magnets to the rotor caused by the centrifugal force generated at high rotor speeds. Generally, when a rotor and its magnets are assembled, the magnets are glued to the rotor, this often being the only means of attaching the magnets to the rotor. At high operating speeds, the centrifugal force generated upon the magnets may overcome the glue bond holding the magnets to the rotor, and damage to the motor may result. If magnets can be more securely retained to the rotor, the operating speed of the motor can consequently be increased as there is less concern regarding the magnets breaking away from the rotor. On rotors where magnets are the same length as the rotor length, rings fastened to the magnets at the ends of the rotor provide a sufficient, additional means of retention.
In addition to retention issues caused by motors operating at high speeds, additional design considerations arise due to the development of high energy product magnets. These magnets may only be manufactured up to a maximum length which is shorter than the required magnet length for some rotors, and methods of magnetizing these magnets place a practical limitation upon their length. Motor designers can compensate for this limitation by stacking shorter magnets lengthwise along the rotor, effectively creating a longer magnet having a more powerful field. Consequently, magnet retention systems solely using end rings as retainers are ineffective when applied to stacked-magnet rotors.
Existing methods of retaining permanent magnets to a stacked-magnet rotor generally include first gluing the magnets to the rotor and, second, employing some additional method of retention, but these methods are either time consuming or degrade motor performance. One such method is a fiber-glass banding process where the rotor and attached magnets are wrapped with a fiber-glass band, then covered with an epoxy coating around the outside of the rotor assembly. This method retains the magnets securely in place, but is expensive, time consuming, and requires additional, accurate tooling. A second method is to surround the rotor with a non-magnetic stainless steel sleeve that fits tightly to the outside of the rotor assembly. However, a stainless steel sleeve introduces additional iron losses at high speeds and requires an increased airgap, thereby lowering the output torque of the motor.